This invention arose from a recognition of the need to produce a lightweight portable printer having the capability of high speed printing of relatively large volumes of characters of graphic information on a sheet with minimal battery requirements.
Portable computers, data collection devices, and other electronic devices often require the use of a printer to produce a "hard copy" of the useful information generated by the devices. This is typically achieved today by the use of various electromechanical printers, which require substantial battery sources or frequent recharging in order to meet the electrical power requirements of the movable print heads, motors, heaters and paper drive assemblies included in such printers. The preferred embodiment of the present device is designed to minimize the mechanical complexity of any printer, to minimize the electrical power requirements of the printer, and to produce a durable printer having unlimited flexibility with respect to the nature of the characters and graphic information that it can produce.
U.S. Pat. No. 4,486,760 to Funada et al. discloses a recording system utilizing a liquid crystal display cell. The basic structure disclosed in the patent pertains to interposing a twisted nematic liquid crystal cell between a light source and a photosensitive recording member. The image generated by the display cell is thereby recorded on the photosensitive recording member. However, in order to overcome the viewing angle dependency of such a display cell, and the light dispersion that takes place within such a cell, the patent teaches the use of a fiber plate including a plurality of optical fibers having axes perpendicular to the plane of the liquid crystal layer as a replacement for one of the conventional glass substrates in such cells. The fiber plate is stated to substantially reduce the distance between the photosensitive recording member and the image light source to produce a clean and high contrast recording.
To expand upon this concept, two limiting considerations are addressed. The first of these factors is the viewing angle dependency of a highly multiplexed liquid crystal display cell, and the effect of this viewing angle dependency on the contrast ratio of the element to be recorded when exposed in a direction perpendicular to the plane of the image. For purposes of this disclosure, contrast is considered to be a non-issue, as technological advances are constantly increasing the effective contrast of liquid crystal displays, and contrast, by definition, can be controlled to a large extent through exposure times, exposure intensity and the chemistry of the recording medium.
The second and more significant factor is resolution. The principal factor influencing resolution in liquid crystal display recording scheme is parallax, and the resulting ambiguity as to the exact location of the edge of an image when projected upon the recording surface, a finite distance away, by light rays coming in at various angles. One method of overcoming the parallax problem is to render the light rays perpendicular to the image to be superimposed upon the recording medium after passage through the liquid crystal display cell. Such a method is described by Funada et al. in U.S. Pat. No. 4,486,760, in which a unique liquid crystal display cell is constructed, substituting a "fiber plate" for one of the glass plates in a conventional liquid crystal display cell.
While the use of a fiber plate as described in U.S. Pat. No. 4,486,760 might be practical in very small scale applications, the cost involved in producing such an assembly for normal full page reproduction of data is prohibitive. Furthermore, with respect to portable printing, the bulk, weight and fragility of such a cell would render it impractical except for stationary applications.
Control of the radiation directed to a liquid crystal display cell in a planar projection scheme is a difficult proposition due to the need to produce uniform amounts of radiation over the entire surface of the liquid crystal display and the recording plane, while simultaneously ensuring substantially parallel incidence of the radiation upon the two planar surfaces. A much more direct approach offered by the present disclosure is to form the image or projection plane in the shape of a cylinder, relying on the radial property of the cylinder to limit dispersion angles in one dimension, and to control dispersion angles at the source through a simple baffle arrangement in the orthogonal direction.
The present solution is to overcome parallax by controlling the direction of the radiation between the source and the liquid crystal display cell, ensuring that the rays that leave the source impinge upon the display plane of the liquid crystal display, and subsequently upon the surface of the recording medium, in essentially parallel fashion.
It is therefore an object of the present disclosure to produce acceptable levels of resolution at the surface of the photosensitive recording medium without resorting to exotic and expensive modifications of current liquid crystal display cell materials.